The present invention relates, in general, to an improved method to control the removal of contaminants from flue gas and, in particular, to a new and useful apparatus and method for controlling removal of contaminants such as sulfur oxides, acid gases, and air toxics from flue gas through the use of a dry scrubber.
Because electric power generating plants and other industries combust fossil fuels such as coal, oil, petroleum coke, and/or waste materials, various contaminants such as sulfur oxides, acid gases, and air toxics appear in the flue gas produced during combustion. Several methods have been employed to remove these contaminants in order to comply with regulatory requirements and emission standards.
One approach that has been used to comply with federal and state emission requirements involves utilizing fossil fuels that are low in sulfur content and other contaminants to reduce the amount of sulfur oxides and other pollutants produced during the combustion process. Increased demand for these fuels results in increased costs due to supply/demand economics, and increased production costs to ship these fuels from distant locations may also be involved. These added costs are then passed on to the end user in the form of higher utility bills.
Other approaches to reduce the sulfur content and decrease other contaminants found in the flue gas employ mechanical and/or chemical processes to reduce the sulfur content in the fuel prior to combustion. However, these processes have proven to be very costly since a high degree of sulfur removal from the raw fuel is necessary to achieve desired reductions of sulfur emissions in the flue gas.
One known method used to reduce sulfur oxide content in the flue gas is to mix dry alkali material with the fuel prior to combustion, or to inject pulverized alkali material directly into the hot combustion gases to remove the sulfur oxides and other contaminants through absorption or adsorption. The major disadvantage of this approach is that such alkali injection usually results in low to moderate removal efficiencies, poor reagent utilization, and increased particulate loadings in the flue gas which may necessitate further conditioning of the flue gas (i.e., humidification or sulfur trioxide injection) when the injection process is conducted upstream of an electrostatic precipitator.
Another known method to reduce sulfur oxides in the flue gas is to use a wet chemical absorption process, known as wet scrubbing, wherein the hot fluid gas is washed in an upflow gas liquid contact device with an aqueous alkaline solution or slurry to remove sulfur oxides and other contaminants. Major disadvantages with this process include the loss of liquid both to the atmosphere (due to saturation of the flue gas and mist carry over) and with the sludge produced in the process. These processes also generally require special materials for construction of the absorber module and downstream auxiliary equipment, such as dewatering and waste water treatment systems.
Another known method for reducing sulfur oxides in flue gases is through a spray drying chemical absorption process, also known as dry scrubbing, wherein an aqueous alkaline solution or slurry is finely atomized (via mechanical, dual fluid, or rotary type atomizers), and sprayed into the hot flue gas to remove the contaminants. A major disadvantage involved with dry scrubbing is that there is a moderate to high pressure drop or loss across the spray dryer gas inlet distribution device, along with a limitation on the spray down temperature, (i.e., how close the temperature of the flue gas exiting the reaction chamber can approach the flue gas saturation temperature) required to maintain controlled operation of the process.
FIG. 1 illustrates a known horizontal, co-current flow dry scrubber, generally designated 10, manufactured by The Babcock & Wilcox Company. Untreated flue gas 12 enters the dry scrubber 10 through a flue gas inlet 14 and travels through the dry scrubber 10 in direction 16. The dry scrubber 10 utilizes a slurry source 18 and an air source 20 to remove sulfur oxides and other contaminants from the flue gas 12. A flush line 22 is also provided on the dry scrubber 10 to recycle unused slurry through the system. A plurality of atomizers 24 spray an atomized aqueous alkaline solution or slurry into the flue gas 12 within a spray absorption region 26. Once the sulfur oxides and other contaminants are absorbed from the flue gas 12 in the absorption region 26, treated flue gas 28 flows through an outlet 30, typically to a particulate collector (not shown).
FIG. 2 illustrates another dry scrubber, this one a known vertical, co-current, down flow dry scrubber 32. Sulfur oxides and other contaminants are removed from the flue gas 12 in the dry scrubber 32 which uses a roof gas distribution device 34 located at the top of the dry scrubber 32, as well as an inlet 36 located at a lower portion of the dry scrubber 32. Sulfur oxides and other contaminants are removed from the flue gas 12 in an absorption region 38. Particles are collected in a hopper 40 located at a lower portion of the dry scrubber 32, and treated flue gas 28 leaves the dry scrubber 32 via outlet 42.
It is thus apparent that a dry scrubber having reduced gas side pressure drop would be of great benefit since lower power consumption continues to be a goal in any emission control installation. The performance of such a dry scrubber can also be improved while minimizing deposition by controlling both the flue gas flow and reagent spray flow in a turbulent mixing zone.